Engines may use a turbocharger or a supercharger to improve engine torque/power output density and minimize fuel consumption. In one example, a turbocharger may include a compressor and a turbine connected by a drive shaft, where the turbine is coupled to an exhaust manifold side and the compressor is coupled to an intake manifold side. However, such a supercharged or turbocharged engine, under low engine speed conditions, may experience torque drop due to a decrease in exhaust-gas mass flow which may subsequently lower a turbine pressure ratio.
One approach to address the issue of torque drop under low engine speed conditions has been to vary valve overlap through variable valve timing, for example. Another approach has been to use an exhaust-gas turbocharger with a small turbine cross-section or by the use of multiple exhaust-gas turbochargers.
The inventors herein have recognized various issues with the above systems. For example, scavenging losses may occur during valve overlap, which may prevent a full charge of fresh air from entering the cylinder and participating in combustion, thus reducing power output. Moreover, the large valve overlap needed to overcome torque deficit at low engine speeds may not be viable in internal combustion engines with high compression ratios, such as diesel engines, due to the proximity of the piston to the valves as it moves toward top dead center. An exhaust-gas turbocharger with a smaller cross-section may be able to generate charge pressure at low exhaust-gas flow rates, but may consequently shift the torque drop further toward lower engine speeds.
One approach that at least partially addresses the above issues is a method and a system, comprising opening at least one exhaust valve to maximum valve lift during a charge exchange of a combustion chamber comprising a piston, actuating at least one intake valve before the piston reaches top dead center of the combustion chamber during the charge exchange to perform an additional intake valve lift such that an intake pressure is greater than an exhaust pressure, then opening at least one inlet valve to maximum valve lift during the course of charge exchange and actuating at least one exhaust valve before the piston reaches bottom dead center during charge exchange to perform an additional exhaust valve lift such that intake pressure is greater than exhaust pressure. In this way, the additional valve lifts may reduce scavenging and/or may provide increased low end torque without increasing exhaust gas temperature, in one example.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.